1. Field of the Invention
This invention relates to a process and apparatus for treatment of hazardous liquid wastes through enhanced ettringite formation, in particular, to control solubility of toxic trace elements such as arsenic, boron, chromium, molybedenum, selenium, and vanadium, as well as other elements that exist as oxyanions in an aqueous solution. More particularly, this invention relates to a process and apparatus for promoting the formation of substituted ettringite and related materials for the purpose of stabilizing potentially hazardous oxyanions as water-insoluble solids.
2. Description of Prior Art
Natural gas industry operations can generate wastes with elevated concentrations of arsenic, boron, chromium, molybenum, selenium, and vanadium. In addition, coal conversion solid wastes from coal gasification and other related processes generally contain elevated amounts of arsenic, boron, molybdenum, and selenium. Arsenic, chromium, and selenium are hazardous elements regulated by the U.S. Environmental Protection Agency; boron is toxic to plants when present in elevated concentrations in irrigation water; and molybdenum is likewise toxic. If mobilized in the environment, these potentially problematic elements are typically present in solution as oxyanions. Ettringite is both an individual mineral, calcium aluminosulfate hydroxide hydrate, Ca.sub.6 Al.sub.2 (SO.sub.4).sub.3 (OH).sub.12.26H.sub.2 O, and the group name for a series of chemically analogous isostructural compounds. One example of a naturally occurring boron--containing ettringite is charlesite, Ca.sub.6 Al.sub.2 (SO.sub.4).sub.2 (B(OH).sub.4)(OH,O).sub.12.26H.sub.2 O. In this compound, one of the sulfate anions has been replaced with a B(OH).sub.4 oxyanion. Thus, these calcium aluminosulfate hydroxide hydrate minerals in which sulfate can be substituted by other oxyanions, such as those listed hereinabove, represent one potential solution for the environmentally responsible handling and disposal of hazardous liquid wastes. The incorporation of these oxyanionic elements into ettringite phases through the addition of ettringite-forming materials at the time of disposal, or by injection after disposal for remediation, is a practical and inexpensive means of mitigating problems with disposal of hazardous liquid wastes containing these trace elements.
Ettringites are naturally occurring compounds. The nominal conditions for the synthesis of ettringite in the laboratory include a pH between 11 and 13 and stoichiometric amounts of calcium, aluminum, and sulfate. Although ettringite can be easily synthesized in the laboratory, nominal conditions for ettringite synthesis do not result in efficient removal of trace amounts of potentially problematic oxyanions such as those listed hereinabove. See Hassett, David J. et al., "Fixation of Trace Elements by Ettringite; a Screening Study", Energy and Environmental Research Center, Mining and Mineral Resources Research Institute, University of North Dakota, Grand Forks, Md., September 1991. In addition, we have determined that, although the formation of ettringite to remove oxyanions occurs naturally, the efficiency of removal is low, on the order of less than 15%. Thus, in order for ettringite formation to be effective in removing oxyanions, the ettringite must be formed in a manner which promotes fixation of substantially all of the oxyanions.
Methods for forming ettringites are generally taught by U.S. Pat. No. 3,992,220 in which white Portland cement is hydrated with water in excess of that needed for hydration, agitated, and subsequently combined with hydrated aluminum sulfate; U.S. Pat. No. 4,002,484 in which a substantially stoichiometric mixture of calcic aluminate and calcium sulfate, formed at a temperature in the range of about 20.degree. C. to 90.degree. C., is hydrated simultaneously, water being added at least in stoichiometric proportions for the reaction and at maximum in an amount such that after the reaction the product is obtained containing 5% (dry) ettringite and 95% (by weight) water; and related U.S. Pat. No. 4,140,540 in which ettringite is formed in accordance with the process of the '484 patent but with enough water to assure having a suspension containing a maximum of 25% dry matter at the end of the reaction, and in subjecting the suspension to moderate stirring of 1 to 5 times the force necessary to maintain a homogenous suspension.
Fixation or encapsulation of radioactive liquid waste by solidifying the radioactive liquid waste within an organic solidifier is taught by U.S. Pat. Nos. 4,671,897, 4,775,495, and 5,202,062. However, each of these references suggests that ettringites are not suitable for use in disposing of radioactive waste by solidification on the basis that the resulting solidified package tends to break, resulting in leakage of the radioactive nuclides.
Related U.S. Pat. Nos. 4,804,521, 4,931,264, and 5,100,643 all teach processes for treating acid gases to remove acid gas components, such as sulfur/oxides, therefrom. The '521 patent and the '264 patent both teach preparation of an aqueous slurry composed of a calcium alkali source and a source of reactive silica and/or alumina, heating the slurry to above-ambient temperature for a period of time in order to facilitate the formation of sulfur-absorbing calcium silicas or aluminates and treating the gas with the heat treated slurry components. The '643 patent teaches preparation of a calcium silicate hydrate sorbent in the form of a semi-dry, free-flowing powder, and treating the gas with the powdery sorbent, such as by injecting the sorbent into a stream of the gas.
U.S. Pat. No. 3,663,252 teaches a process for stabilization of aluminous cements in which a substance is added to an anhydrous cement mixture to prevent the formation of cubic aluminate and free water in the cement.
U.S. Pat. No. 3,720,609 teaches a process for treating waste sludges by adding to the sludges materials capable of producing aluminum ions, lime and/or sulfate-bearing compounds and hardening the resulting composition. Hardening occurs over time by formation of calcium sulfo-aluminate hydrates that is, ettringites. Similarly, U.S. Pat. No. 4,132,558 teaches a process for treating sludges or drainages containing chromium compounds by treatment of the sludge or drainage with a rapid hardening cement containing calcium haloaluminate; and U.S. Pat. No. 4,741,776 teaches a process for treating waste streams containing heavy metals by treating the material with flue dust and bonding it to form a solid mass with cement powder.